This invention relates to a method and apparatus for curing a coating applied to an elongated filament. More particularly, it relates to a method and apparatus for maintaining a coated optical fiber at a relatively low temperature during the curing operation.
Glass optical waveguide fibers must exhibit high strength in order to withstand the stresses which are encountered while incorporating them into protective sheathing or cable, while installing the cable, or during use. While such fibers are typically quite strong as drawn, this strength is rapidly degraded by surface defects which are introduced into the fiber through handling or otherwise. To preserve the strength of a newly drawn fiber, it is conventional to apply a protective coating to the fiber immediately after it is drawn.
The coated fiber usually passes through an ultraviolet curing oven which employs a lamp that generates infrared radiation in addition to the ultraviolet energy needed for curing. If an excessive amount of infrared radiation reaches the coating, it can cause evaporation of the surface of the coating material. In a two-stage coating apparatus, the primary coating is usually subjected to ultraviolet energy for curing prior to the application of the secondary coating. If the primary coating is not maintained at a sufficiently low temperature when the fiber enters the second coater, the viscosity of the primary coating will be so low that variations in diameter of the first applied coating can result. Some defects caused by an excessively high coating temperature can be detected by observing a section of coated fiber under a microscope where the surface of the primary coating can be seen through the secondary coating. The primary coating should appear as two parallel lines, indicating uniform diameter. If the primary coating is too hot when the coated fiber passes into the secondary coater, the primary coating surface may appear as two undulatory lines. Also, the temperature of the coated fiber can be measured by a non-contact fiber temperature measuring device. The maximum permissible temperature for conventional UV curable coating materials is about 80.degree. C.
When drawing fiber at a relatively low rate, we cooled the coating material in the curing oven by passing the coated fiber through a transparent cylinder through which inert gas was flowed. For example, a draw rate of about 5 m/sec required the flow of about 12 1/min of nitrogen to cool the coating. When the draw rate was increased to 10 m/sec, the power of the ultraviolet light source had to be increased in order to adequately cure the coating, and the nitrogen flow rate had to be correspondingly increased. An inordinately high rate of coolant gas flow is expensive and can cause excessive vibration of the fiber whereby it is deflected from its intended position in the cavity where the ultraviolet light is most highly focused. Also, fiber vibration can affect coating concentricity in the preceding coating apparatus.